1. Field of the Invention:
This invention pertains to control mechanisms for nuclear reactors, and more particularly to hydraulically operated drive mechanisms for a control rod having a positive mechanical latch arrangement for holding the control rod in a withdrawn position.
2. Description of the Prior Art:
In accordance with the prior art, certain power reactors are controlled by a combination of chemical shim systems and rod cluster control systems. In pressurized water reactors of the type exemplified, the chemical shim system typically consists of adding boric acid to the reactor coolant. Boric acid has the effect of lowering the effective multiplication factor of the nuclear core to slightly above 1.0 so that the nuclear chain reaction is just capable of maintaining itself and does not become supercritical. There is significantly more boric acid present in the reactor coolant at beginning of core life than at the end of core life due to the differing amounts of fissile material present in the core during those times, respectively. Conceivably a pressurized water nuclear reactor may be completely controlled with a chemical shim system; however, the time factor involved in changing the concentration of boric acid makes this method of control impractical. As a result, a boric acid chemical shim is usually assisted by a rod cluster control system which permits rapid changes in reactivity of the nuclear core. The rod system is a mechanical system which generally comprises 16 to 20 control or neutron absorbing rods situated to be moved axially within cooperating guide tubes in selected fuel assemblies of the core. In the earlier prior art, all of the control rods associated with each of the fuel assemblies were attached to a single spider-like hub which in turn was attached to a drive shaft. Thus, all of the control rods were operated simultaneously and because of the relatively large worth of a single control rod assembly, the control rods were operated in discrete steps over the entire distance of travel.
As indicated and explained in detail in U.S. Pat. No. 3,519,535 issued July 7, 1970 to Erling Frisch and Harry Andrews, A Nuclear Reactor, and assigned to the assignee of the present invention, reactors utilizing rod control systems or other incrementally movable control elements have several limiting characteristics. The worth of each control cluster is too great to be used for suppressing radial flux peaks. Partial insertion of a cluster can cause sever perturbations in the axial flux distribution and can lead to xenon cycling. As further explained in pending application Ser. No. 53,206, filed July 8, 1970 of E. Frisch and H. Andrews, Reactor Refueling Method, now Pat. No. 3,775,246 granted Nov. 27, 1973, and assigned to the present assignee, an optimum control system would accordingly have two primary characteristics. A wide disposal of individually movable low worth absorber rods; and no control configuration wherein certain control rods are partially inserted. Such a control system would result in appreciable savings due to more efficient usage of nuclear fuel. In this regard, a highly reliable drive mechanism which is capable of positioning a plurality of two position control elements is necessary to render such a desirable control system practical. A relatively large number of drive mechanisms would, however, be necessary; further, they must not be so large that they cannot be mounted side by side on a reactor vessel. A prior art solution to these problems is disclosed in U.S. Pat. No. 3,607,629 issued Sept. 21, 1971 by E. Frisch et al, Drive Mechanism For Control Elements, and assigned to the present assignee.
The most recent prior art then, discloses a hydraulic control rod drive mechanism which utilizes the substantial pressure available in pressurized reactors to move the absorber or control rods relative to stationary fuel assemblies with which they are associated. The hydraulic mechanism allows independent movement of individual control rod drive shafts having one or more absorber rods attached thereto, to be fully withdrawn from or fully inserted into the core. Each mechanism has provision to accommodate a multiplicity of control rod drive shafts. A number of electromagnets are mounted to the hydraulic mechanism; one electromagnet being associated with each control rod drive shaft. A fully withdrawn control rod is held in this position by actuation of the respective electromagnet. Thus, in the prior art a mechanism is disclosed which allows a reactor to be controlled by a large and diverse pattern of low worth two position control rods and therefore to more nearly achieve the full potential of its fissile fuel.
Even with these most recent developments, in the prior art, namely, hydraulic control rod drive mechanisms, the method of operation for full length control rods is, however, not considered satisfactory for part length control rods which are also required in today's large nuclear reactors. Part length control rods are utilized to trim the axial power distribution of the core and to prevent divergence of the xenon cycling within the core. In a typical large nuclear reactor, eight part length control rod assemblies each containing 20 individual control rods are interspersed throughout the core. For purposes of comparison, the same reactor example would also use 53 full length control rod assemblies, with each assembly containing 20 individual control rods. A part length control rod contains absorber material only in the lower part of its length. For example, in the same large reactor previously illustrated, the lower three feet of a control rod having a total length of twelve feet contains neutron absorbing material. Withdrawal of a part length control rod moves the absorber section from the lower to the upper part of the core and is therefore not totally removed or withdrawn from the core as is the case with the regular control rods. Tripping or rapidly inserting a part length control rod may actually increase core reactivity. Simultaneous tripping of several part length control rods may then result in an undesirable increase in reactor power level even though all full length control rods are tripped at the same time. Such an event is a distinct possibility in the prior art in reactors equipped with hydraulically operated control rod drive mechanisms by accidental deenergizing of an electromagnet holding coil bus.